I thought some might be interested in the following excerpt I received from Amazon.com re their sales rank for each book on their used book site:
"Yes, sales ranks are based both on Amazon.com and Marketplace purchases i.e. comparison between all books we sell (new and used).
Sales ranks are updated every hour of every day around-the-clock for each of millions of items. An item's sales rank is a reflection of its entire sales history at Amazon.com. In order to keep these lists fresh, useful, and up-to-date, recent sales are weighted more heavily than sales that occurred in the distant past."
"Yes, sales ranks are based both on Amazon.com and Marketplace purchases i.e. comparison between all books we sell (new and used).
Sales ranks are updated every hour of every day around-the-clock for each of millions of items. An item's sales rank is a reflection of its entire sales history at Amazon.com. In order to keep these lists fresh, useful, and up-to-date, recent sales are weighted more heavily than sales that occurred in the distant past."